In many types of instruments such as oscilloscopes and physiological monitoring devices, it is desirable to provide a floating input amplifier which is electrically isolated from the remainder of the instrument. The problem becomes apparent where the object under test is at a different voltage potential than the measurement instrument itself, which is usually at ground potential in accordance with recognized electrical standards. For example, in measuring the ripple on a high-voltage supply to evaluate the efficiency of a regulating circuit, it would be desirable to "float" the input stage at the D.C. level of the supply and thereby avoid a wide difference of potential connected across the input terminals.
A floating input amplifier is particularly important in physiological monitors, where the object under test is a human body. It is essential that any shock hazard be minimized, that is, that power supply potentials within the monitoring device or line voltage from the A.C. mains not reach the patient in the event of an instrument malfunction. Also, it is important from the standpoint of providing an instrument capable of closely monitoring signals being generated from a source which may be totally insulated from ground or any other potential, or which may be subjected to the potentials of other clinical devices connected thereto.
There have been many previous attempts to provide a floating input amplifier for an electronic instrument. One scheme utilized a transformer-isolated amplifier powered by batteries. Not only are batteries bulky and their power limited, but they also require recharging or replacement. Another scheme utilized a pair of transformers; one for an energy path and one for a signal path.